Here's a book perfectly timed for the season of self-flagellation. The cover shows the face of a woman who has been gorging on chocolate, the shameful evidence trickling from her vampiric lips. It's an image calculated to make anyone who has over-indulged of late, in whatever way, commit to a regime of monkish abstinence. Enough of sex, drugs and profiteroles: now's the time for fasting and press-ups. And where better to start than with a sermon on the sins of the flesh?

Disappointingly, however, Paul Martin doesn't believe that pleasure need be bad for us. In fact, his book ends with a list of recommendations for the wily hedonist - ways to enjoy ourselves without feeling guilty about it. Far from condemning the pursuit of pleasure, Martin shows how unavoidable it is: encoded in our minds and genes. And rather than rail against the licentiousness of contemporary life, he commemorates the sensation-seekers of centuries past, whose excesses make our own seem tame in comparison.

Though he doesn't deny the dangers of addiction, his account of drug use down the ages quietly defuses tabloid hysteria. Cannabis? Queen Victoria took it to relieve her period pains. Cocaine? Freud prescribed it to patients and used it himself for relief from migraines. Opium? The users range from Marcus Aurelius to Robert Louis Stevenson. Alcohol? Churchill routinely drank a bottle of champagne a day, whereas Hitler was teetotal - nuff said.

As with drugs, so with sex. Throughout history, church and state have worked to prohibit almost everything except procreative sex within marriage, but science suggests that recreational sex (in all its varied forms) is good for us. Orgasms relieve stress, reduce the risk of heart disease and prostate cancer, and activate the immune system: a study of middle-aged men found that those who had two or more orgasms a week had a mortality rate 50% lower than men who had orgasms less than once a month. Martin also speaks up for masturbation as a "biologically mainstream form of behaviour". Bears do it. Deer do it. Even dolphins at the zoo do it. Baden-Powell thought no boy scout should do it ("it quickly destroys both health and spirits; he becomes feeble in body and mind, and often ends in a lunatic asylum"). But today even the idea that masturbation is a sad alternative to real sex is under challenge: one survey found that women who reported masturbating also have more sexual intercourse; another that masturbation is more prevalent among those who are better educated and from a higher social class.

The chapters on sex are not without interest - I confess to never having heard of the anal violin before, a 19th-century contraption designed to create erotic vibrations when played in situ. But it's chocolate that stirs the author's passion. If chocolate has a bad press, he argues, that's only because the stuff most people in the UK eat is a poor sugar-and-fat substitute; the real thing - with at least 60% cacao solids - is health-giving, mood-enhancing and richly sensuous, thanks in part to its subtle psychoactive properties. It's also, he insists, non-addictive. True, it has long been associated with decadence and carnality: the Marquis de Sade once gave a ball at which the guests were fed chocolate pastilles containing an aphrodisiac ("All who ate them were seized by shameless ardour and lust"), and chocolate also spiced up a bawdy episode in his One Hundred and Twenty Days of Sodom. But a more telling contribution may be the flavonoids (antioxidant chemicals) in chocolate, which reduce the risk of cardiovascular disease.

The pleasure principle is, on the whole, a sound one, then: having what you like is fine so long as you don't have too much of it in one go. But as a scientist by training, Martin is also keen to explain how the principle works in practice - the key being the way that pleasurable experiences release a neurotransmitter substance called dopamine in the nucleus accumbens region of the brain. The neurobiology is complex, but Martin keeps it bracingly simple, even when elaborating on terms such as anhedonia (the inability to feel pleasure), acedia (aka accidie or taedium vitae) and ataraxia (a state of serenity). Numerous experiments with rats are cited. What they mostly illustrate is a truth that writers and philosophers arrived at a millennium or two ago: that pleasure and desire are different things, since the former can be satisfied but the second cannot.

Other important questions arise. The distinction between desire, pleasure, happiness and satisfaction, for instance. And the conundrum of what constitutes the opposite of pleasure - is it boredom, misery or pain? John Wilmot, aka the poet Rochester, thought it the first. "I hate still-life," he said, by way of justifying a career dedicated to boozing, fighting and fucking. By contrast, Montaigne found no joy so great as passing a kidney stone and recovering "the beauteous light of health". Casanova's take on pleasure is different again: he had no regrets about having spent his life as a libertine, but recalling his experiences was as important to him as having them: "What pleasure in remembering one's pleasures," he wrote. And then there's Shakespeare, on our capacity for enjoying what we dislike and vice versa: "Why lov'st thou that which thou receiv'st not gladly, / Or else receivs't with pleasure thine annoy?"

Martin's approach is less complicated. A little of what you fancy does you good, he suggests, while advancing a personal choice of modest pleasures, among them walking, gardening, cooking, fishing, napping, sitting in silence and having lunch. All are legal, and can help improve one's life, with none of the torture of self-improvement. Janis Joplin and Errol Flynn would find them dull. But they're unlikely to kill you. And they work out much cheaper than health spas.